tools: fix use-after-free mkcodecache warning

Call `v8::Platform::ShutdownPlatform()` to fix a Coverity warning about
the `v8::Platform` instance being deleted when it's still in use.

PR-URL: https://github.com/nodejs/node/pull/27332
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

tools/code_cache/mkcodecache.cc

@@ -58,5 +58,6 @@ int main(int argc, char* argv[]) {
     out.close();
   }
 
+  v8::V8::ShutdownPlatform();
   return 0;
 }